Carbon fiber-reinforced composite materials are excellent in terms of strength, stiffness, conductivity and the like. Carbon fiber-reinforced composite materials are widely employed in, for example, aircraft structural members, windmill blades, automobile outer panels and computer applications such as IC trays and chassis (housings) of laptop computers, and the demands for such carbon fiber-reinforced composite materials have been increasing every year.
One mode of a carbon fiber-reinforced composite material is a heterogeneous material obtained by molding a prepreg constituted by carbon fibers, which are reinforcing fibers, and a matrix resin. In this case, there is a large difference in the physical properties between the direction of the reinforcing fiber alignment and other directions. For example, it is known that since the impact resistance, which is represented by the resistance to a drop impact, is dictated by the delamination strength quantitatively measured in terms of interlayer plate-edge delamination strength or the like of carbon fiber-reinforced composite material, a mere improvement in the strength of reinforcing fibers does not lead to a drastic improvement of the impact resistance. In particular, a carbon fiber-reinforced composite material comprising a thermosetting resin as its matrix resin reflects a low toughness of the matrix resin and thus has a property of being easily broken under a stress applied from a direction other than the direction of the reinforcing fiber alignment. Therefore, a variety of technologies have been proposed for the purpose of improving the physical properties of a carbon fiber-reinforced composite material capable of withstanding a stress applied from a direction other than the direction of the reinforcing fiber alignment,
As one of the technologies, a prepreg in which resin fine particles are dispersed in the surface portion has been proposed. For example, there is proposed a technology for providing a high-toughness composite material having good heat resistance with the use of a prepreg in which resin fine particles composed of a thermoplastic resin such as nylon are dispersed in the surface (see Patent Document 1). Further, there is also proposed a technology for allowing a carbon fiber-reinforced composite material to exhibit toughness at a high level by using a combination of a matrix resin, whose toughness is improved by an addition of a polysulfone oligomer, and resin fine particles composed of a thermosetting resin (see Patent Document 2).
However, in those carbon fiber-reinforced composite materials that are obtained by applying the above-described technologies, although the impact resistance is improved, an insulative resin layer is formed between the layers. Consequently, the conductivity of the resulting prepreg in the lamination direction is very poor.
In view of this, as a method of improving the interlayer conductivity, there have been proposed methods in which metal particles and conductive particles such as carbon particles are incorporated in a matrix resin in advance (see Patent Documents 3 and 4) and a method in which a conductive film is blended with a matrix resin in advance (see Patent Document 5).